16 INHERITANCE OF CHARACTERISTICS IN DOMESTIC FOWL. 



between offspring and parent. The index is larger than it would otherwise 

 be because it is measured with an average of the parents and these parents 

 assortatively mated. But this instance is, in any case, an interesting 

 example of strong inheritance of a quantitative variation. 



"What, it may be asked, is the relation of these facts to the general 

 principle that inheritance is through the gametes? Why, when a gamete 

 with the median element unites with a gamete without that element, does 

 the zygote develop a soma that in some cases shows a nine-tenths median 

 and sometimes a one- tenth median element? We have seen that the Y 

 comb is a heterozygous form due to imperfection of dominance of the median 

 element; but why this variation in the perfection of the median element? 

 This is probably a piece of the question, why any dominance at all. We find, 

 in general, that the determiner of a well-developed organ dominates in the 

 zygote over the determiner of a sUghtly developed condition of that organ 

 or its obsolete condition. It is as though there were in the zygote an inter- 

 action between the strong and the weak form of the determiner, and the 

 strong won; but sometimes the victory is imperfect. In the specific case 

 of comb the interaction between median and no-median leads to a modifi- 

 cation, weakening, or imperfection of the median element, and this weaken- 

 ing varies in degree. Sometimes the weakening is inappreciable when the 

 comb is essentially single ; sometimes it is great, and the result is a comb in 

 which the median element is reduced to one-half; sometimes, finally, the 

 determiner of median comb is so completely weakened by its dilution with 

 "no-median" as not to be able to develop, and we have the cup comb with 

 only a trace of the median element. Nevertheless, such a cup comb is 

 heterozygous and produces both single-combed and PoHsh-combed germ- 

 cells. Thus the variation in the extent of the median comb seems to point 

 to variations in relative potency of the median comb over its absence. 



